PROJECT SUMMARY Exciting results from the heterochronic parabiosis model suggest the presence of rejuvenation factors in the circulation that restore youthful characteristics to aged cells and tissues. Identification of robust anti-geronic factors that mediate rejuvenation effects is a key to translating the potential of heterochronic parabiosis into clinical applications. Comparative un-biased proteomic analyses followed by western blot validation of serum samples led us to identify candidate ?conserved? anti-geronic factors that decline with age in mice and humans, including PEDF (SERPINF1) which is known to play a critical role in stem cell self-renewal, maintenance and survival in mice. The overall goal of the proposed project is to identify and validate circulating anti-geronic factors among our top conserved candidate factors by utilizing both in vivo and in vitro heterochronic parabiosis experimental systems. This work will be organized to address the three critical research questions: 1) Do candidate anti-geronic factors restore youthful properties to aged cells and tissues in vitro and in vivo?; 2) Do candidate anti-geronic factors mediate the beneficial effects of heterochronic parabiosis on aged tissues and of heterochronic transplantation on aged stem cells?; and 3) Do candidate anti-geronic factors alter age-related cellular epigenomic profiles in vitro and in vivo? To achieve our goals: We will prioritize conserved candidate anti-geronic factors by measuring their ability to phenocopy young serum in vitro (Aim 1); We will test for necessity of anti-geronic factors to mediate beneficial effects of heterochronic parabiosis on aged tissues in vivo (Aim 2); and We will address whether the candidate anti-geronic factors prioritized in Aim 1 and validated in Aim 2 act to alter age-related cellular epigenomic profiles both in vitro and in vivo as determined by genome-wide analyses (Aim 3). Our long-term goal is to assess whether anti-geronic factors, alone or in combination, delay or reverse the age-associated functional decline of cells and tissues, thereby revealing potential targets for therapeutic intervention.